Sugar is refined by this invention from both phases or forms in which it appears in industry and trade, as raw crystals or as syrups, i.e. concentrated solutions formed by evaporation and possibly other treatments of the concentrated juice from sugar beets or sugar cane. The impurities are also removed from two phase mixtures of sugar and water; e.g. solid raw sugar which carries a thin film of liquid phase molasses, also a massecuite of solid crystals in syrup. These impurities are largely concentrated in the solvent layer which is decanted at the end of the contacting operation as a solvent-extract liquid.
Impurities which must be removed may be classified as those which are very soluble in water, e.g. invert sugars, and those which are often present in much smaller amounts and are less soluble in water as oils, fats, and waxes. Both classes of impurities are washed or extracted by solvents from the solid raw sugar which may contain water in as low as 0.2% to 0.4% and total solids of 99.6% to 99.8%, a massecuite of larger water content, or a syrup having a water content of from 20% to 45% and a total solids (Brix) of 55% to 80%. The solvent has a molecule having a weight of less than 62 and a hydroxyl group; and it is by itself completely soluble in water. A co-solvent -- acetone -- also completely soluble in water -- is used in selected proportions. The impurities are thus removed from a water-sugar mixture in one or two phases containing water between 0.2% and 45%.
Known methods allow the recovery of the residual solvent and the working up of the purified sugar crystals or syrups to give substantially pure crystalline sugar or pure "liquid sugar" syrups; also the stripping of solvent from the extract layer; also the separation of the impurities remaining into their components -- invert sugars, acids, fats or oils, waxes, chlorophyll, and molasses rich in vitamins. Some of these impurities have significant commercial values which are lost or destroyed in conventional refining processes. The solvent mixture is reused with little loss.
While this process will be used in conjunction with many known steps in sugar processing, its novelty resides in counter current flows and contactings by liquid washings or extractions of raw sugar crystals or of raw or otherwise impure sugar syrups of high concentration with mixtures of two volatile, water miscible, oxygenated solvents.
Cane juice is expressed and concentrated usually near the tropical plantation in a "central". During the final evaporation and "graining", a "strike" of crystals is formed. These grow and are separated to give a "raw" sugar containing several percent of impurities. Most of this raw sugar is shipped to be refined in the country where it is to be consumed.
By the novel process of this invention, essentially one or two counter current washings, extractions, or counter-current contactings of a stream of syrup by a stream of solvent, the concentrated juice may be converted to refined sugar at the central, and may then be shipped in bulk or otherwise to world markets; or refined syrups may be made from molasses or raw sugar; or the raw sugar, as conventionally made may be purified by washing i.e. counter-current contacting of a stream of it and a solvent mixture which can be produced on the site by fermentation of residual molasses. In another embodiment, cane juice is concentrated to 60-80% solids or Brix and shipped in bulk tankers for refining by the novel process in the country of use. Similarly syrups or molasses from beet juice may be purified without the large number of conventional physical and chemical treatment steps.
Sugar syrups, for the present purpose, are sugar solutions which are in the process of being refined. A molasses is a syrup resulting from a refining process which contains some or most of the impurities of the original syrup. For identification, a syrup molasses or raffinate molasses is one resulting from the separation of sugar crystals or syrup from what has previously been a less pure aqueous sugar-water mixture, while a solvent extract molasses is one resulting from a solvent extraction, either immediately after removal of the solvent -- or after an evaporation-graining which may have allowed a strike and separation of sugar crystals.
The first step in the conventional refining of raw sugar is the washing off with aqueous sugar liquors of impurities present in a film of molasses on the yellow raw sugar. This is the so-called "affination" process and is done in a so-called "mingler", a horizontal scroll-conveyor-mixer, and then a centrifuge, to separate as much as possible of the syrup containing the impurities away from the crystals. This same mingler may be used to wash out the impurities from a heavy massecuite, a syrup containing a large amount of sugar crystals. With either raw sugar or massecuite it is desirable to operate with the solvent sluicing out extraneous solids as dirt, fiber, etc. from the sugar. These solids are separated then mechanically by settling or filtration from the solvent-extract liquid discharged.
For many years sugar refiners have tried to use ethanol in the affination of raw sugar without success, and for the liquid-liquid extraction of other solids, i.e. various impurities, away from a sugar syrup in a final molasses.
For example, Vazquez in U.S. Pat. No. 2,000,202 treated a concentrated molasses with a nearly anhydrous ethanol mixed with a second liquid such as ethyl acetate. This combination dissolved the impurities and precipitated or crystallized the sugar out in a mass or massecuite of crystals. The alcohol and impurities were removed as an extract molasses containing the impurities; and the sugar crystals were then later dissolved with more dilute alcohol from the insoluble impurities which remained.
Alcohol has been found to be a poor solvent for many of the impurities while it is, as noted in Vazquez, when somewhat diluted, a good solvent for the sugar -- thus no industrial use has been reported of systems based on its use as: (a) an affination solvent, (b) an extraction liquid for impurities from a syrup or molasses, or (c) for precipitating crystals of sugar and washing them, then dissolving them as suggested in U.S. Pat. No. 2,000,202.
Bohrer U.S. Pat. No. 3,174,877 used methanol with 1 to 5% of a hydrocarbon to decolorize raw sugar in an affination, and showed that ethanol was definitely unusable for this purpose. His solvent was not chosen to remove other impurities of raw sugar, with which 3,174,877 was unconcerned.
Leonis U.S. Pat. No. 1,558,554 dried molasses and treated this with glacial acetic acid for 2 to 24 hours during which time the impurities evidently went into solution, the sugar was precipitated; and the impurities remained in the mother liquid.
Othmer U.S. Pat. No. 3,325,308 washed sugar crystals with pure methanol or pure acetic acid, separated the impurities in an extract molasses, removed the solvent therefrom; and then, out of this molasses, extracted with acetone the oils, fats and waxes for which the acetone has an excellent selectivity.
Acetone was also used with molasses from other sources to extract the oils, fats, waxes therefrom and to leave the highly water soluble inverts with the raffinate stream of sugar syrup.